How do I know what size to make retina images?

How do I know what size to make retina images?

References

Reference NO 1

http://stackoverflow.com/questions/15188204/how-do-i-know-what-size-to-make-retina-images

Reference NO 2

http://stackoverflow.com/questions/15575581/how-to-find-that-my-retina-device-is-using-2x-image

Reference NO 3

http://line25.com/tutorials/how-to-create-retina-graphics-for-your-web-designs

Reference NO 4

http://www.kylejlarson.com/blog/2012/creating-retina-images-for-your-website/

Reference NO 5

http://www.karelia.com/sandvox/help/z/Retina-Optimized_Images.html

Reference NO 6

https://discussions.apple.com/thread/3812804?start=0&tstart=0

Reference NO 7

http://www.allanberger.com/post/49938151321/how-to-make-your-apps-retina-ready

Reference NO 8

http://www.studiopress.com/design/css-background-size-graphics.htm